1. Field of the Invention
The present invention relates generally to liquid heaters and particularly swimming pool and spa heaters which transfer heat from products of combustion to water as it is being circulated through a heat exchanger.
2. Description of the Prior Art
A constant problem associated with the operation of conventionally known swimming pool and spa heaters is that after the heater is shut off, water circulation through the heater is also shut off and water left in the heat exchanger inside the unit continues to be heated by residual heat stored in the mass of the unit. The amount of this residual heat is quite often sufficient to cause the temperature of the remaining water in the heat exchanger to rise close to the boiling point before the heat can be dissipated to a point where the water temperature will begin to lower. Because the residual water in the heater can reach much higher temperatures than does the circulating heated water as it is returned to the swimming pool or spa, the last several feet of piping leading to the header of the heater unit on conventionally known heaters and the header couplings are made of copper or other high temperature piping material providing heat resistance characteristics sufficient to address the problem. This is a disadvantage because the use of plastic piping and couplings would offer significant manufacturing cost savings. Additionally, the high header temperatures place undesirable space restrictions on the location of the heater unit.
Another problem associated with such heaters is that they are often elevated above the level of the body of water in the swimming pool or spa. If a leak occurs in the water conduit system associated with the heater, the water in the heater will automatically drain and leave the heater dry. If the heater is fired while dry, a "dry-fire" will occur which may cause a melt-down of the heat exchanger before any of the typical safety controls in the heater are able to respond. A common practice which has been employed to address this problem is to incorporate a water trap through an extension to the heat exchanger or manifold which prevents the water from completely draining under such circumstances. If a "dry-fire" occurs in a heater having such a trap, the liquid trapped in the manifold or heat exchanger will rapidly convert into steam, the presence of which will more quickly activate the heater safety controls to shut down the heater.